The present invention generally relates to a communication system for transmitting and receiving encrypted digital signal samples. The present invention more particularly relates to an encryption system for use in such a communication system for providing encrypted digital signal samples from digital signal samples to be encrypted.
In the transmission of data or voice intelligence, digital techniques are often employed to enhance or improve transmission quality and effectiveness. One application for such digital techniques is in cordless portable telephone systems wherein the amplitude of analog signals representing speech are quantized and multiple-bit digital samples representing the quantized speech amplitudes are used to modulate a radio frequency carrier. The radio frequency carrier is transmitted over a radio frequency channel for reception at a distant point, such as a base station. At the receiving point, the digital samples are extracted from the carrier and are converted to analog signals which are applied to a speaker, for example, for reproducing the original speech.
Because such transmissions are conducted in the radio frequency spectrum, they are available for reception by any one having suitable receiving equipment. Hence, such transmissions are not secure transmissions. In order to secure such transmissions, the digital samples are encrypted or transformed pursuant to a predetermined encryption code. As a result, the received encrypted transmissions will be unintelligible unless the receiving equipment incorporates decryption apparatus for decrypting the transmissions in a manner complimentary to the encryption code.
While encryption and decryption systems of the prior art have been generally successful in securing radio frequency digital transmissions, they have exhibited some deficiencies. For example, such systems can require alteration of the transmission bit rate requiring more complicated equipment to receive and decrypt the digital samples than would otherwise be necessary. Also, encryption systems of the prior art can degrade reception quality by not providing an accurate reconstruction of the original analog signals. Further, prior encryption systems can be inflexible in not allowing the encryption code to be altered during transmissions to render the transmissions more secure.